I've written a Javascript engine back in 2000. I came away from that with a distaste for dynamic typing. It seems easier to write code with dynamic typing, but I wouldn't give it many points for robustness. JS has many odd behaviors if an unexpected type was passed to a seemingly simple function.
The implications you're suggesting about power, memory, and speed, or even predictability in semantics, depend on the particular language spec they implemented (it was not standard ecmascript), how they implemented, and how that implementation was used within the larger system.
It seems to me like just knowing it was javascript (for some definition of javascript) is not enough.
Probably because they couldn't find low level programmers, it isn't easy to find people with good professional experience in C today, especially if you pay NASA wages.
I just spent a few minutes looking into their actual usage of it and, ime, it has all the indications of being a reasonable decision.
Here's a bit more info from [0]:
> It is my opinion that NASA settled on the Nombas engine, after extensive research of their own into all the options available at the time, not so much because of the JavaScript language itself (although the familiarity of the language was a bonus), but because of the solid and robust nature of the ScriptEase engine and the tools that went with it.
[0]: http://brent-noorda.com/nombas/us/index.htm